The present invention relates to a mixing device for mixing at least two substances, preferably a liquid monomer and a pulverulent polymer, for manufacturing bone cement, whereby the two substances are mixed in a mixing space in a mixing container, preferably in partial vacuum, and whereby the substances are provided separated from each other in one container for each substance.
In medicine, the implant technique has made substantial progress in recent years. Particularly the application methods for bone cement during implant of prostheses as well as the use of bone cement as place holder in the restorative surgery, have made astounding progress.
During the time of origin of bone cements, said bone cements were mixed in a cup or bowl at atmospheric pressure and temperature, kneaded by hand and formed into "sausages" and put into the defects to be filled and finally compressed by hand. These methods were for a long time the only possible application method for bone cement.
Then, an improvement in the form of an application through a cartridge was developed. Here too, the bone cement was mixed at normal atmospheric conditions, but then put in a cartridge and forced out through an orifice therein by means of a mechanical power transmission apparatus (pistol).
The next progress in the application method was the complete elimination of hand contact with the bone cement, whereby mixing was carried out in a special cup or bowl from which the mixed cement was transferred directly into the cartridge.
At these times, the term of life of implanted prostheses was mainly dependent on the skill of the surgeon. The term of life of the implants was dependent on the perfection with which the cement was mixed and applied. The term of life was not dependent on the quality of the prostheses (except for the commonly known shaft breakages), since the term of life for bone cement was far below that of the prostheses. Here, the physicians and scientists acknowledged an essential development deficit and stucked the principal reasons for the failure of the bone cements. In principal, two main reasons were found for the relatively short term of life of the bone cements. First, there was an incomplete mixing of the monomers with the polymers, which lead to low and various primary strengths within the cement casing or mantle around the prostheses and which because of uncombined and polymerized monomer and polymer components lead to toxic problems and inflammatory tissue reactions. Secondly, examinations indicated mechanical inhomogenities in the cement caused by enclosed air bubbles which have a substantial negative influence on the fatigue strength of the bone cement. However, coarse defects because of enclosed air bubbles also caused a premature breakage of the cement casing or mantle.
After a substantial reduction or elimination of infections by the integration of antibacterial active substances in the bone cements, which also prolonged the term of life of cemented prostheses, it was clearly necessary also to improve the mechanical properties of the bone cements in order to thereby further increase the term of life of the prostheses.
Acknowledging these circumstances, L. Lidgren developed a method for mixing bone cements and at the same time reducing the pores therein. In accordance with this method, the bone cements were mixed under vacuum in the application cartridge. Thereby, the monomer was put into the cartridge, the polymer powder filled therein and the cartridge sealed with a mixing device. After generating a vacuum, the polymer was mixed with the monomer under vacuum. The components were allowed to complete their reaction, also under vacuum, and first thereafter the bone cement was discharged from the cartridge through a snorkel or discharge pipe by means of a pistol. This system for mixing and applying bone cement is commercially available under the name "Optivac" from the company MIT AB and corresponding studies have proven its efficiency and reliability.
Experiments have shown that thanks to such a mixing procedure it is possible to substantially reduce the porosity (number and size of the air bubbles) of the cement matrix and, thus, the mechanical strength of a cement mixed accordingly.
A literature survey regarding the abovementioned mixing of bone cements under vacuum follows after this description.
From the patent literature, U.S. Pat. Nos. 4,463,875 and 4,973,168 as well as EP patent application No. 0 194 508 should be mentioned.
U.S. Pat. No. 4,463,875 defines a mixing device, whereby two substances to be mixed are packed up beforehand in the mixing device separated from each other. The substances are brought in contact with each other and thereafter mixed. In order to do this, the package is loaded from the outside with reciprocating movements, so that the package is deformed and the substances thereby mixed. A drawback is here that with these mixing movements, no bone cement with the required quality can be obtained. Furthermore, a complex container design is required for carrying out these mixing movements.
In U.S. Pat. No. 4,973,168 there is described a mixing device, wherein one of the substances to be mixed is packed up in the mixing device beforehand, while the other substance is packed up in a separate container independent of the mixing device. For mixing the substances, the container is emptied into the mixing device, whereafter the substance from the container is mixed with the substance already present in the mixing device by means of an inner mixing means. A drawback herewith is that a separate container is required for one of the substances and that there is a risk for that the separate container is not completely emptied or that a part of the substance in the separate container is spilled out and, thereby, the bone cement produces does not fulfill the quality demands set.
EP patent application No. 0 194 508 describes a mixing device for mixing other substances, whereby no vacuum is generated.
The object of the present invention is to provide a mixing device which renders it possible to improve the present mixing technique and to exclude errors in the preparation before mixing.
This is arrived at according to the invention by means of the characterizing features of the claims.
By means of these features it is particularly provided for that no transfer of substances is required and spill out of at least one substance is avoided, and that the substances also are sufficiently mixed for obtaining a bone cement with the required quality.
Hereby, without additional manipulations, the cement components can be located sterile ready for mixing in the mixing container, which, inter alia, leads to a reduction of the technical manipulations and minimization of the environmental danger and the amounts of waste. Furthermore, the contamination risk through the bone cement is reduced and no additional amounts of waste are produced. Errors in the order of actions taken during mixing are eliminated.